1. The Field of the Invention
The present invention relates to a method and apparatus for assuring that a pacemaker is properly programmed so that bipolar operation will not be enabled on an implanted unipolar lead and thereby cause loss of capture.
2. Prior Art
The typical implanted cardiac pacemaker operates by supplying missing stimulation pulses on a pacing lead attached to the heart. The so called "R" wave can be sensed by a lead in the ventricle, which lead can be used for purposes of pacing. An additional lead can contact the atrium to sense the "P" wave, if desired. In programmable pacemakers, the fixed rate at which the pulse generator will produce pulses may be selected from among a variety of optional rates, for example, from forty to one hundred beats per minute. It is desirable to have as many rates available as is practical for two reasons. First, it enhances the physician's ability to match the pacemaker to the patient so as to avoid angina and to coordinate the fixed rate with the patient's normal sinus rhythm. Second, the higher the rate, the shorter the lifetime of the pacemaker's battery. By the same token, it is desirable to have a number of different pulse intensities available for selection. Pulse intensity is programmed either by adjusting pulse width or pulse amplitude. It is generally desirable to minimize the pulse intensity to conserve power. The stimulation output of the pulse generator is applied to the heart via two electrodes, namely, a cathode and an anode. Typically the cathode for ventricle pacing is placed adjacent to the tip of an elongated insulated pacing lead which extends pervenously into the right ventricle of the heart. The electrical return path of the anode can be achieved in two different ways. The case of the pulse generator can be used as the anode. In this system, since only one electrode is located at the end of the pacemaker lead, the lead is called an "unipolar" lead. In the other system, the anode is located near the tip of the pacemaker lead displaced and insulatively separated from cathode, which lead is referred to as a "bipolar" lead.
In the past, cardiac pacemakers and pacing leads were available in either unipolar or bipolar configurations and the anode location was irreversibly selected at implantation. If a unipolar lead was implanted and it was determined that sensing could be more reliably obtained with a bipolar lead, or if there were muscular twitching in the vicinity of the pulse generator, the patient had to undergo a further operation to replace both the lead and the pulse generator. Attachment of an unipolar lead to a pacemaker having bipolar operational capability would leave a ring electrode contact unconnected. This condition can be sensed. A similar situation would occur should the ring portion of a bipolar lead develop an open circuit.
A more recent trend has been to provide programmable pacemakers which can be programmed for either unipolar or bipolar operation. There are generally four conditions which can be programmed, namely: unipolar sensing with unipolar pacing; unipolar sensing with bipolar pacing; bipolar sensing with unipolar pacing; and bipolar sensing with bipolar pacing. However, there remains the problem of assuring that the pacemaker will not be programmed for bipolar operation when, in fact, a unipolar lead is implanted and attached to the pacemaker.
The present invention constitutes a method and apparatus for assuring that, at the time of reprogramming an implanted pacemaker, only a program compatible with the type and condition of the implanted lead will be enabled. Normally, a pacemaker capable of bipolar operation is initially programmed for unipolar operation. After implantation, it is necessary to assure there is a functional bipolar lead connected to the pacemaker before initiating a bipolar program. The present invention generates a test signal in a logic and control circuit and passes the signal through an impedence to the electrodes. A sensing amplifier detects the presence of noise, which is either the injected noise signal or actual noise of a fairly high intensity. If the sensing amplifier does detect a signal, and the patient is not in a high noise field, this indicates that a unipolar condition probably exists, either because a unipolar lead was implanted or an implanted bipolar lead is not fully operational, and only unipolar pacing should be programmed.
The present invention teaches programming to a pacing and sensing mode in which a high impedance test signal is applied to a sensing node that will be high impedance (with respect to pacemaker ground V.sub.DD) if an improper or inoperative electrode for the desired program is connected to the pacemaker. A test signal can be selected from any one of a number of signals including, but not restricted to, signals similar to noise with a requirement that the number of such signals exceed a predetermined minimum number in order to make a determination of the lead configuration. The signal would preferably be generated from a master micro-circuit of the pacemaker. It would also be possible to use clock pulses as a signal source with the additional requirement that the clock pulses be of sufficient amplitude and periodicity as to be readily detectable.
The normal sequence of events is to implant the lead and the pacemaker into the patient, monitor the patient's condition, and then reprogram the pacemaker at later times, as required by the patient's changed condition, by non-invasively transferring parameter value data from an external device, called the programmer, to the pacemaker implanted in the patient's body. A number of programming systems have been successfully employed in commercially available cardiac pacemakers, including magnetic programming and radio frequency programming, to reprogram an implanted pacemaker. Pacemakers are usually initially programmed by the manufacturer to a unipolar sensing and pacing condition. There are programming sequences which are followed when changing from this initial condition. For example, when going to bipolar operation, the present invention goes through an intermediate step of bipolar sensing with unipolar pacing to ensure capture during the test. There is the possibility, due to the time between implanting and the time of reprogramming, that there may be an oversight as to the exact nature of the lead which has been implanted in the patient. There also is the possibility of the ring portion of a bipolar lead becoming open after implantation. One of the problems addressed by the present invention is assuring that this reprogramming is compatible with the type/condition of the lead which is implanted.